#ifndef PRPPELTHREADPOOL_H
#define PRPPELTHREADPOOL_H

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>

namespace blk {

    class ThreadPool {

    public:

        ThreadPool(size_t);

        template<class F, class... Args>

        auto enqueue(F &&f, Args &&... args)

        -> std::future<typename std::result_of<F(Args...)>::type>;

        ~ThreadPool();

    private:
        // need to keep track of threads so we can join them
        std::vector<std::thread> workers;
        // the task queue
        std::queue<std::function<void()> > tasks;

        // synchronization
        std::mutex queue_mutex;
        std::condition_variable condition;
        bool stop;
    };

    // the constructor just launches some amount of workers
    inline ThreadPool::ThreadPool(size_t threads) : stop(false) {
        for (size_t i = 0; i < threads; ++i)
            workers.emplace_back(
                    [this] {
                        for (;;) {
                            std::function<void()> task;

                            {
                                std::unique_lock<std::mutex> lock(this->queue_mutex);
                                this->condition.wait(lock, [this] {
                                    return this->stop || !this->tasks.empty();
                                });
                                if (this->stop && this->tasks.empty()) {
                                    return;
                                }
                                task = std::move(this->tasks.front());
                                this->tasks.pop();
                            }

                            task();
                        }
                    }
            );
    }

    // add new work item to the pool
    template<class F, class... Args>
    auto ThreadPool::enqueue(F &&f, Args &&... args) -> std::future<typename std::result_of<F(
            Args...)>::type> {
        using return_type = typename std::result_of<F(Args...)>::type;

        auto task = std::make_shared<std::packaged_task<return_type()> >(
                std::bind(std::forward<F>(f), std::forward<Args>(args)...)
        );

        std::future<return_type> res = task->get_future();
        {
            std::unique_lock<std::mutex> lock(queue_mutex);

            // don't allow enqueueing after stopping the pool
            if (stop) {
                // throw std::runtime_error("enqueue on stopped ThreadPool");
            }

            tasks.emplace([task]() { (*task)(); });
        }
        condition.notify_one();
        return res;
    }

    // the destructor joins all threads
    inline ThreadPool::~ThreadPool() {
        {
            std::unique_lock<std::mutex> lock(queue_mutex);
            stop = true;
        }
        condition.notify_all();
        for (std::thread &worker: workers) {
            worker.join();
        }
    }

}

#endif //PRPPELTHREADPOOL_H
